It's official, federal scientists say 2012 was the hottest year on record for the Lower 48 States. In fact, the average shattered the previous record set in 1998.

Here's NPR science correspondent Richard Harris.

RICHARD HARRIS, BYLINE: We didn't need to wait for the end of the year to know that 2012 was miserably hot and miserably dry. Still, Jake Couch, at the National Climatic Data Center, put it on the record today.

JAKE COUCH: Two thousand twelve marked the warmest year on record for contiguous U.S., with the year consisting of a record warm spring, the second warmest summer, fourth warmest winter, and a warmer than average autumn.

HARRIS: Couch says the record wasn't even close. In more than 100 years of record keeping, the average stayed within a range of 4 degrees Fahrenheit. But 2012 was a full degree outside of that range and it was dry to boot. Many people suffered through a bad year of drought.

Couch says you may not be hearing so much about it now in the dead of winter...

COUCH: But we are still seeing impacts from the drought such as low water levels along the Mississippi, causing commercial shipping problems; near low-water levels in the Great Lakes. So we are still in the midst of this drought. It is not over and I foresee that it's going to be a big story moving forward in 2013.

HARRIS: One huge part of the United States did escape record heat and drought, Alaska was actually cooler and a bit wetter than average. But globally, the year was one of the 10 hottest on record.

Deke Arndt, at the Climate Data Center, says even though 2012 doesn't top the list globally don't shrug it off.

DEKE ARNDT: It's very easy to get very enamored with the records. You know, the biggest, the most, the highest. We've spent a lot of time in this kind of top 10 territory globally temperature-wise.

HARRIS: The world's climate scientists overwhelmingly agree that the Earth is getting warmer because human activities are pouring huge quantities of greenhouse gases into the atmosphere. But Jake Couch says it's not a simple matter to say how much of the heat last year was caused by human activities and how much was part of the normal ups and downs.

COUCH: Climate change has had a role in this. The contiguous U.S. temperature has been increasing and is still increasing, and local variability and regional variability did play a role. But it's hard for us to say at this time what amount of the 2012 temperature was dependent on climate change and which part was dependent on that local variability.

HARRIS: Given the trends, it's very likely that we'll see a lot more heat records in the years to come.